1. Field of Invention
This invention relates to a demodulation, in particular, to a frequency modulation or frequency shift keying (FM/FSK) demodulator.
2. Related Art
FIG. 1 shows a basic structure of conventional FM/FSK demodulator. It contains: a multiplier 21 and a phase shifter 11. The function of the FM/FSK demodulator is to convert a frequency-varying input signal into an amplitude-varying output signal. FIG. 2 is a schematic view showing the curve of the input frequency variation versus the output amplitude variation. The curve is generally called an S-curve. The input frequency variation and the output amplitude variation in the labeled region 11a have a linear relation. This is the working range of the FM/FSK demodulator. The shifted phase of the phase shifter 11 is required to be very precise to have larger demodulation gain of this circuit. To increase the strength of output signal and the noises of the FM/FSK demodulator, the conversion slope of the input frequency variation to the output amplitude variation can be increased. In practice, any device has its error coefficient. If the shifted phase of the phase shifter 11 has any deviation, it will result in the following problems:
(1) The conversion curve of the FM/FSK demodulator deviates from its normal linear region. This will distort the demodulation output signal, and the system cannot function normally. (2) In the narrow-band FM/FSK demodulating system, the conversion slope of the input frequency variation to the output amplitude variation has to be tuned up so as to obtain a sufficient output signal. However, it has very little tolerance to the device errors. Therefore, adjustments have to be performed during the production process. This complicates the manufacturing process. (3) In the low voltage and low power conditions for the narrow-band FM/FSK demodulating tasks, in addition to the larger conversion slope of the input frequency variation to the output amplitude variation, the limited work voltage also has small tolerance to direct current (DC) offsets. Even if adjustments are made in the production process, it is still difficult to increase the yield.
In U.S. Pat. No. 6,798,678, the patent uses an indirect means to adjust the shifted phase of the phase shifter for compensating the phase. It has two similar phase shifting circuits. The first phase shifting circuit is adjusted using a phase-locked loop (PLL), thereby indirectly adjusting the second phase shifting circuit. This is more complicated in the circuit structure. In fact, the adjusted results cannot be completely consistent. It also needs an extra oscillatory circuit that causes additional interference.
In U.S. Pat. No. 6,847,255, the phase shifter is integrated in a single chip, without any consideration of production errors. Therefore, it is only suitable for wide-band demodulation. With little error tolerance, it cannot be applied to narrow-band demodulation. Consequently, it has few applications.
Therefore, how to provide a FM/FSK demodulator that has a phase compensating function and can be integrated into a single chip is an important subject in the field.